“Disappointing” results from autologous bone marrow transplants in HIV-infected patients has focused new emphasis on finding out where and how HIV successfully hides once effective therapy is discontinued, even treatment as drastic a treatment as a transplant.

In research published in the journal Cell online October 24, Johns Hopkins infectious disease experts say the amount of potentially active, dormant forms of HIV hiding in infected immune T cells may actually be 60-fold greater than previously thought. This latent reservoir is now widely recognized as the major barrier to curing HIV-1 infection and is the subject of an intense international research effort.

In 2008 and 2010, two male HIV-infected patients received autologous bone marrow transplants to treat lymphoma, a form of blood cancer, effectively providing them with a new immune system from a noninfected donor. Both individuals, referred to as the “Boston patients” because they were treated in Boston, continued to take their antiretroviral therapy after the transplant to prevent any residual virus hiding in their bodies from infecting their new blood cells.

Virus levels in their blood became almost undetectable for an extended period of time following the bone marrow transplants, lying low for eight months following the procedures. Earlier this year, when they had been apparently virus-free for several years, both men elected to halt their antiretroviral therapy. The patients agreed to stop taking their HIV medications to determine whether the drugs were controlling the infections or whether it was the transplant of healthy donor bone marrow cells that had caused remission.

But earlier this month, Timothy Henrich, M.D., of Brigham and Women's Hospital in Boston, who had treated the men, reported at the Sixth International Workshop on HIV Persistence during Therapy, held in Miami, that the virus had reemerged in the patients and both men had resumed combination ARV therapy after stopping it for 12 and 32 weeks, respectively.

While ARV (or antiretroviral therapy, ART) therapy suppresses HIV levels in the body by disrupting viral replication in blood cells—usually CD4 cells and/or by blocking viral interaction with target cells to prevent infection—no therapies reach and destroy dormant, nonreplicating viruses remaining in reservoirs in the body. Upon cessation of treatment, HIV emerges to reestablish active infection, which is what investigators say occurred in the Boston patients.

But as one adult male patient with HIV infection—the “Berlin” patient—appeared to have been successfully treated in Germany, scientists had been encouraged that the Boston patients might remain in remission.

The Berlin Patient

In 2009, German hematologist Gero Hütter performed a bone marrow transplant on an HIV patient being treated with a bone marrow transplant for leukemia. Hutter recalled that individuals who carry the CCR5 mutation in both copies of the gene are less likely to contract the virus. Among the 80 matches he located as potential bone marrow donors in the German Bone Marrow Donor Center, one donor carried the mutation. Hutter reasoned that replacement of the patient’s immune cells with cells lacking the CCR5 receptor might render him less likely to be susceptible to HIV infection.

The Berlin patient, Timothy Brown, moved to San Francisco about two or three years ago and “graciously agreed to participate extensive studies there,” said UCSF’s Steven G. Deeks, M.D., professor, UCSF Positive Health Program at San Francisco General Hospital and an AIDS researcher at the University of California at San Francisco Medical School. He added, “We did everything possible to find enough biologic specimens to see if there was any virus left, and we distributed these tissues throughout the world to people who were expert at finding low levels of virus. At the end of the day, I think most of us came to the conclusion that there may be remnants of the virus, but proving that they are real is not possible with current methodologies. Importantly, we found no evidence that there is any virus around that could replicate.”

But it is unlikely that bone marrow transplants will be used to treat HIV. In bone marrow transplants, a patient’s own blood-forming stem cells are largely obliterated and then replaced. The painful procedure renders patients exhausted and extremely vulnerable to infection until the new stem cells take up residence in the bone marrow and restore the destroyed immune system.

This patient had received a stem cell transplant while being treated for relapsed leukemia. But Dr. Hutter in this case had located marrow donors with the CCR5 mutation. CCR5 is one of two co-receptors that allow HIV-1 entry into human cells after the virus binds to the cell surface CD4 receptor. A minority of healthy individuals who are homozygous for this naturally occurring 32 base pair deletion in the CCR5 gene produce a defective gene product that makes CD4+ T cells nonpermissive to HIV-1 infection.

This patient was taken off all antiretrovirals prior to his conditioning regimen and subsequent transplant. At the time of the original report, HIV-1 had remained undetectable for 20 months. A follow-up report described no detection of HIV-1 RNA or DNA in blood, cerebrospinal fluid, or gut mucosa.

In commenting on the implications of this outcome, Jeffrey Laurence, M.D., the chief scientist at amfAR, the Foundation for AIDS Research, spoke with The Body in 2009, where he explained that individuals homozygous for the delta32 CCR5 mutation, technically “you are resistant to getting infected by most forms of HIV.”

Following the first successful transplant, Brown relapsed with leukemia about a year later and required a second transplant. “Now in this time,” Dr. Lawrence explained, “the patient had no HIV viral load and had good T-cell levels despite the fact that his antiviral drugs were stopped. His virus didn't come back and his T-cell levels remained high—that's the miracle of this patient.”

Shock and Kill

Despite the relapse, experts still find the Boston cases exciting because, they say, they are another step in the long and so-far-fruitless search for a cure. Dr. Deeks says they offer encouragement to ambitious future projects to genetically re-engineer infected patients' cells to be infection-resistant. At least two teams are already experimenting with variants on this idea.

He adds, “The big barrier to a cure is that there are cells that lie dormant, survive for decades, and contain hidden within them the virus. That is the primary limitation, and the primary goal of cure research is to get rid of it. One approach that is quite attractive to many investigators is this concept of ‘shock and kill.’ One uses a drug to shock the cells to force the virus out and then one hopes the cell will die, or one modifies the immune system to make sure all virus-producing cells die.” But, he says, “We are at the beginning of that, and we are just now starting with the shock part.”

In the shock-and-kill approach, he explained, the idea is to use a drug to shock the cells to force the virus out and then one hopes the cell will die, or one modifies the immune system to make sure all virus-producing cells die.

Dr. Deeks notes that several groups have presented data showing that “certain drugs that alter the DNA, that modify the chromatin that increase the cell's capacity to transcribe DNA, can, in fact, force the virus out of its hiding place. Whether that leads to any meaningful outcome, no one knows, but it proves the concept. At least the shock part, shocking it out of its hiding place, is theoretically possible.”

Recent reports suggested for example that histone deacetylase inhibitors (HDAC inhibitors) including vorinostat, belinostat, and givinostat, may be effective inducers of latent HIV expression. These inhibitors, scientists say, may offer a “unique opportunity to deplete the latent reservoir. An additional benefit is the anti-inflammatory properties of HDAC inhibitors including downregulation of HIV-1 coreceptor expression.”

But Anthony Fauci and colleagues at NIAID, in 2012, published studies demonstrating that HDACs do not induce HIV production in the latent viral reservoir of aviremic individuals suggesting that alternative therapeutic strategies may be necessary to eliminate HIV in the latent viral reservoir.

The ultimate goal is, says Dr. Deeks, is finding a cure for HIV infection and AIDS disease. “We don't really want to have a handful of Berlin patients,” he says. “That's not going to change the world. We all want to cure people in a big way with drugs that could be given for a few months everywhere in the world. And the best prevention is to cure someone. If you are cured, you're not going to infect others.”

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